The invention relates to an apparatus and a method for reducing the current drawn while starting a single-phase AC asynchronous motor.
The limiting of the starting current of inductive or capacitive loads is a problem that has been known for a long time and whose solution has been aimed at in widely differing ways. An additional starting resistor is often used during the starting phase, and limits the current that is drawn. Similar measures are known during switching off, in order to avoid current surges or voltage spikes.
In addition to industrial users, a large number of private households load the public mains power supply system by drawing excessive currents for appliances such as refrigerators or air-conditioning systems. The compressors in appliances such as these are switched on and off frequently, so that the public mains power supply system is frequently loaded. This results in undesirable mains voltage fluctuations.
In order to reduce the starting current, motors have until now been used with a main and an auxiliary winding, with a capacitor being connected between the voltage source and the auxiliary winding. During the starting phase, the capacitor and the auxiliary winding were switched on by means of a relay, and were switched off once the starting phase had been completed. Apparatuses such as these have the disadvantage that switching the relay on and off results in unacceptable current surges.
DE 28 48 281 discloses a control apparatus for reducing the starting current, in which the main winding and auxiliary winding are initially entirely connected to the mains voltage in a first time period. The maximum current thus flows through the main winding and auxiliary winding. This leads not only to an undesirable switch-on current surge, but to the rotor being practically stalled by the maximum current in the rest state. Alternatively, it has also already been proposed for an NTC thermistor to be arranged in such a way that both the current through the main winding and the current through the auxiliary winding are reduced at the moment of switch-on. This on the one hand reduces the maximum current through the main winding, but also on the other hand limits the current through the auxiliary winding, thus making the starting behavior worse. In known arrangements, the NTC thermistor is bridged, and rendered ineffective, after the end of the starting process.